1. Field of the Invention
The present invention relates generally to communicating between control equipment on the earth's surface and a subsurface drilling assembly to command downhole instrumentation functions. In particular, the present invention relates to communicating, detecting, and decoding instructions to the drilling assembly via pressure pulse signals sent from a surface transmitter without interrupting drilling. More particularly, the present invention relates to apparatus and methods for using the pressure pulse signals to detect torsional vibrations, or stick-slip, in the drill string while drilling.
2. Background and Related Art
In hydrocarbon drilling operations, the drill bit and other components of the bottom hole assembly (BHA), and even the drill string itself, are subjected to conditions which increase wear and degradation of these expensive components. One such condition is called “stick-slip,” or torsional vibration of the BHA. Stick-slip is a downhole condition where torsional vibrations have increased because the bit and BHA are experiencing increased friction and drag at the bit, causing the bit to stop rotating. Once the bit has stopped rotating, torque tends to build up in the drillstring. The torque buildup causes the energy in the drillstring to increase until it overcomes the drag friction between the bit/BHA and the earthen formation, which frees the bit momentarily until the drag friction overcomes the rotational energy in the drillstring again. This causes a periodic motion called stick-slip.
Stick-slip is a major contributing factor to excessive bit wear. Torsional vibration can have the effect that cutters on the drill bit may momentarily stop or be rotating backwards, i.e., in the reverse rotational direction to the normal forward direction of rotation of the drill bit during drilling. This is followed by a period of forward rotation of many times the rotation per minute (RPM) mean value. The effect of reverse rotation on a cutter element may be to impose unusual loads on the cutter which tend to cause spalling or delamination of the polycrystalline diamond facing of the tungsten carbide cutter.
If it is known that stick-slip is occurring in the BHA, it may be possible for the operator of the rotary drilling system, at the surface, to reduce or stop the vibration by modifying the drilling parameters, for example by changing the speed of rotation of the drill string (RPM) and/or the weight-on-bit (WOB). However, it is currently difficult to detect at the surface torsional vibration which is occurring in the BHA, and several techniques have been developed to address the problem of detecting the onset of stick-slip of the BHA.
One such method includes the use of downhole RPM data to detect stick-slip. Typically, the average surface and downhole RPM of the drilling assembly is 60 to 150 RPM's. In the event of excessive rotational vibrations, or stick-slip, the downhole RPM's can reach 3 to 5 times, or higher, the average surface RPM's. Downhole RPM data is a direct measurement to detect stick-slip. Devices or tools may be placed downhole to measure RPM, the most common of which is the magnetometer. Using the earth's magnetic field as a reference, the magnetometer can measure how fast the BHA is rotating, and then it is possible to calculate the RPM.
Another prior art method for detecting stick-slip is the use of surface torque data. The surface torque data, when charted as a function of time, will exhibit a periodic motion. The period of the surface torsional vibration will be the same as the period for the downhole torsional vibration, and thus the period may be used to detect stick-slip. Such a method is described in U.S. Pat. No. 6,227,044 to Jarvis, hereby incorporated herein by reference for all purposes.
Additional devices for detecting destructive downhole vibrations include Sperry-Sun's DDS™ Drillstring Dynamics Sensor and Drilsaver™ Real-Time Torsional Vibration Monitor.
However, these prior art methods have associated problems. One problem is the need for additional sensors, such as rotational sensors or magnetometers, or mechanical devices with moving parts. Thus, it would be desirable to use equipment already present in the drilling assembly to detect stick-slip, as well as inherent phenomena associated with the drilling process.
Sometimes, the drilling assembly may include a system for communicating between the surface equipment and the subsurface drilling assembly. Downlink signaling, or communicating from the surface equipment to the drilling assembly, is typically performed to provide instructions in the form of commands to the drilling assembly. For example, in a directional drilling operation, downlink signals may instruct the drilling apparatus to alter the direction of the drill bit by a particular angle or to change the direction of the tool face. Uplink signaling, or communicating between the drilling assembly and the surface equipment, is typically performed to verify the downlink instructions and to communicate data measured downhole during drilling to provide valuable information to the drilling operator.
A common method of downlink signaling is through mud pulse telemetry. When drilling a well, fluid is pumped downhole such that a downhole receiver within the drilling assembly can meter the pressure. Mud pulse telemetry is a method of sending signals by creating a series of momentary pressure changes, or pulses, in the drilling fluid, which can be detected by a receiver. For downlink signaling, the pattern of pressure pulses, including the pulse duration, amplitude, and time between pulses, is detected by the downhole receiver and then interpreted as a particular instruction to the downhole assembly.
For a more detailed description of mud pulse telemetry, and an improved downlink telemetry system, see U.S. Patent Application Publication No. 2003/0016164 Al to Finke et al., application Ser. No. 09/783,158, which was filed on Feb. 14, 2001 and published on Jan. 23, 2003 (the “'158 application”), hereby incorporated herein by reference for all purposes. The '158 application discloses a downlink telemetry system that can be used without interrupting drilling and without interrupting uplink communication such that simultaneous, bi-directional communication is achievable if the uplink and downlink signals are sent at different frequencies. Moreover, the '158 application discloses an algorithm for filtering and decoding the downlink signals. The algorithm determines the time intervals between pulse peaks and decodes the intervals into an instruction.
The stick-slip motion previously described causes pressure fluctuations or pulses downhole. As described above and in the '158 application, the mud pulse telemetry system of the drilling assembly uses mud pulses to communicate. The stick-slip pressure pulses and the telemetry pulses may have very similar frequencies, therefore the noise created by the stick-slip pressure pulses may interfere with proper telemetry signaling.
An objective of the present invention is to use the already present mud pulse telemetry system and its associated pressure while drilling (PWD) sensor to detect the pressure fluctuations created by stick-slip.
Another objective of the invention is to act upon the detection of stick-slip to adjust certain drilling parameters and thereby improve the drilling operation.
Yet another objective of the invention is to improve the reliability of the downlink system by detecting the “false” downlink pulses induced by stick-slip. An aspect of this objective is to filter out such false downlink pressure pulses created by stick-slip.
The present invention overcomes the deficiencies of the prior art.